The present invention relates generally to methods for the production of alkali metal hydroxides, and more specifically, to methods for the electrochemical synthesis of caustic soda without the customary co-production of chlorine.
Alkali metal hydroxides are manufactured in the United States to the extent of approximately 36,500 tons/day, almost entirely by the electrolysis of aqueous brine solutions. In addition to sodium hydroxide the electrochemical synthesis results in the co-production of chlorine. The electrolysis of brine can be shown by Equation I as follows: EQU 2NaCl+2H.sub.2 O.fwdarw.2NaOH+Cl.sub.2 +H.sub.2 (I)
Unlike alkali metal hydroxides, chlorine produced at the anode of an electrolytic cell in stoichiometric quantities to sodium hydroxide has experienced a declining market because of environmental problems. For example, use of chlorine by the pulp and paper industry has been declining because of traces of dioxin formed in paper products; chlorine in the treatment of sewage and water has been shown to lead to the production of toxic organo-chlorine compounds; compounds like the chlorofluorocarbons and methyl chloroform have been found to be destructive to the earth's protective ozone layer, and certain chlorine-containing pesticides have been shown to be toxic to biological systems. Consequently, it is expected that the declining demand for chlorine will continue to weaken in the approaching decades. By contrast, the demand for alkali metal hydroxides, like caustic soda is expected to remain strong.
Accordingly, in view of the declining demand for chlorine and the absence of economical routes for its destruction or safe storage there is a growing need for new and more economical processes for the manufacture of high purity alkali metal hydroxides which do not also produce halogens
A number of methods have been developed for the production of alkali metal hydroxides without the simultaneous production of chlorine. While most methods are effective in eliminating the problems associated with the co-production of chlorine most have not been viewed as commercially acceptable because of various shortcomings, e.g. inefficient consumption of power, inability to produce a sufficiently pure grade of caustic soda and/or co-production of other less desirable products For example, one of the earliest methods for the production of caustic soda without the co-production of chlorine was the so called "lime-soda" process based on the following reaction EQU Ca(OH).sub.2 +Na.sub.2 CO.sub.3 .fwdarw.CaCO.sub.3 +2NaOH (II)
The lime-soda process has several shortcomings. It is difficult to carry out to full conversion; the caustic soda is impure and the process is energy inefficient, particularly if there is any attempt to recycle the calcium by thermal decomposition of the carbonate to oxide.
U.S. Pat. Nos. 3,963,592 and 4,561,945 disclose processes for the production of sodium hydroxide and hydrogen at the cathode by salt splitting methods in which electrochemical cells employed are equipped with hydrogen depolarized anodes for oxidation of hydrogen to form protons. U.S. Pat. No. 3,963,592 provides for the oxidation of elemental hydrogen at the anode to hydrogen ions which in turn can react with the chloride ions in the brine electrolyte to form hydrochloric acid. If desired, elemental chlorine may be formed by the oxidation of chloride ions at the anode in which case hydrogen is not fed to the anode.
Like U.S. Pat. No. 3,963,592, U.S. Pat. No. 4,561,945 also relates to the production of alkali metal hydroxide and hydrogen at the cathode and acid at a hydrogen consuming anode. The principal object of the '945 patent is to provide a recycling process for large quantities of sodium sulfate by-product generated in the production of rayon by converting it to caustic soda and sulfuric acid used in manufacturing rayon. Thus, while U.S. Pat. No. 4,561,945 mentions several salts which may be electrolyzed in the synthesis of alkali metal hydroxides all are associated with the simultaneous co-production of acid, and in particular sulfuric acid, as shown by the following equation III: EQU Na.sub.2 SO.sub.4 +3H.sub.2 O.fwdarw.2NaOH+H.sub.2 SO.sub.4 +1/2O.sub.2 +H.sub.2 (III)
The electrochemical synthesis of alkali metal hydroxides with the co-production of acid has significant shortcomings not recognized by the above U.S. patents, and in particular U.S. Pat. No. 4,561,945. With electrochemical cells having hydrogen depolarized anodes oxidizing elemental hydrogen to H.sup.+ in the co-production of acid there is also a competition of H.sup.30 with the sodium or other alkali metal ion in membrane selectivity. In membrane separated two compartment electrochemical cells having an anolyte side and a catholyte side, as the acid concentration in the anolyte side increases the hydrogen ion prevails over the metal ion. In the case of the electrolysis of sodium sulfate (U.S. Pat. No. 4,561,945) the anolyte will typically have a pH&lt;1. This causes a reduction in alkali metal hydroxide current efficiency and higher power consumption/ton of product produced.
U.S. Pat. No. 4,561,945 discloses the use of a two membrane/three compartment type electrochemical as an alternative to the two compartment cell. The center compartment of the three compartment cell receives the sodium sulfate electrolyte protecting the carbon based gas diffusion anode from the deleterious effects of sulfate ion and sulfuric acid produced in the process. However, a two membrane/three compartment type cell has significant shortcomings, namely higher capital costs, elevated cell voltages and greater power consumption due to increased iR loss. A voltage penalty of &gt;0.5 V can occur in a three compartment electrochemical cell which translates into a 25 to 50 percent increase in power consumption over similar two compartment cells. Hence, while it would be more desirable to employ a two compartment cell in methods of making alkali metal hydroxides without the co-production of chlorine, methods proposed heretofore providing for the co-production of acid have meant significant trade-offs in terms of higher capital and operating costs, including life expectancy of cell components.
Accordingly, there is a need for a more economical and energy efficient method for the electrochemical synthesis of alkali metal hydroxides without the co-production of chlorine and acid.